Platinum is considered one of the best electrocatalyst for low temperature reactions in a H2/O2 fuel cell. Metallic nanoparticles are of great interest because of the modification of properties observed due to size effects, modifying the catalytic, electronic, and optical properties of the monometallic nanoparticles. The polyol method has been reported to produce small nanoparticles as the final product, easily changing composition and surface modifiers. Modifications in temperature influence the reaction by changing the stabilization of the nanoparticles of Pt formed, the surface modifiers, and the nucleation rate of the reduced metallic atoms. The platinum nanoparticles were synthesized by this method and then deposited on a highly orientated pyrolytic graphite (HOPG) substrate. The samples then were heated in a H2 flow. Platinum nanoparticles also were obtained on HOPG in 0.01 M H2Cl6Pt and 0.1 M HCl solution electrochemically by open circuit deposition. The density of particles as well as their dimension depend in this case on the time of immersion in the hexachloroplatinic solution. Pt nanoparticles formed for both methods are mainly spherical. Characterization of these systems has been studied by high angle annular dark field (HAADF) technique and transmission electron microscope (HRTEM).